I know that a lot of runners stock up on carbs before a run. What is the benefit of this? I've heard some people say that you'll "hit a wall" if you don't take in some carbs before running. I'm assuming this is like hitting a plateau in your running ability where you can't progress any further. Is this true?

I'm on a ketogenic diet (low/no-carb) normally, but it seems I definitely perform better when I have some carbs before a run. Why do carbs make such a big difference during exercise while ketone bodies are fine for fuel all the rest of the day?

4 Answers
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"Hitting the wall" isn't so much a plateau as "precipitous fatigue and loss of energy" (Wikipedia). It happens when you run out of glycogen (the storage for of carbohydrate in your body). When I was training for a marathon a couple years back, I went out on a 16 mile run without bringing a source of carbs along. For my first 10 miles (~1h 20min) I had an average pace of 8:21 minutes/mile. After that I started to hit the wall and my pace dropped to 10:40 min/mile. After about 3 miles like that, I was totally depleted had to walk the last 3 miles home. There was just no more energy available. Eating carbs before (and during, if it's really long) helps prevent this by topping my glycogen stores. Many athletes even carb load the week before an event to maximize glycogen stores.

Every nutrition textbook I've read has recommended a high carb diet as definitively the best plan for endurance athletes (Here's the relevant section in the Nutrition textbook my university uses). However, I found an article with a differing viewpoint. According to the author, people on a high protein diet can maintain their endurance performance if given enough time to adapt to using ketones. This may take 3-4 weeks. However, apparently if you wax and wane on your ketogenic diet (sometimes eating carbs, sometimes not), your body wont adapt; you have to be consistent. The author also explains that there are a couple things you have to be careful about when exercising on a ketogenic diet:

Ketogenic diets cause diuresis (water loss) which can take minerals with it. If you're on this kind of diet, you may need to supplement your electrolytes. A scary running-related health issue that pops up in the media every summer is hyponatremia, which is caused by losing too many electrolytes.

If you don't have enough dietary protein to meet your exercise needs, your body will start drawing protein from its "stores," i.e. your muscles (discussed more here). Just because you're on a high protein diet doesn't automatically mean you're consuming enough protein, since the point is often to reduce total calories so you can lose weight. The author recommends you consume 1.2 – 1.7 g of protein per kilogram body mass, and says that if protein intake exceeds 25% of your caloric intake there are negative side effects (protein yields 4 Cal/g, so to figure about how many grams of protein you're allowed, calculate 0.25 * [total Calories consumed] / [4 Cal/g]).

Given all this information, I can think of one additional concern. To prevent your body from tapping into your muscles to get energy during prolonged endurance, you would have to eat protein during exercise.Protein is known to delay gastic emptying (it stays in your stomach for longer), so it's not pleasant to eat during exercise.

The author also squeezes in one final caveat at the end:

Anaerobic (ie, weight lifting or sprint) performance is limited by the low muscle glycogen levels induced by a ketogenic diet, and this would strongly discourage its use under most conditions of competitive athletics.

If I recall correctly, you do interval training, which may push you into the anaerobic zone. If this is the case, it sounds like a ketogenic diet would negatively affect your performance. I'm having a hard time finding a source to explain why this is the case, but maybe it's because anaerobic exercise uses glucose as its primary energy source (you rely more on fat during aerobic exercise and at rest), so large amounts of it would have to be synthesized from protein; maybe the body can't keep up with the demand.

Are there only benefits or can I also take too many carbs?
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Ivo Flipse♦Apr 5 '11 at 8:02

Why aren't my body's regularly produced ketone bodies good enough if I'm on a ketogenic diet? The only time I seem to need any carbs at all is when I run, the rest of the time I'm on a low/no-carb diet and I have no shortage of energy.
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Nathan Wheeler♦Apr 5 '11 at 14:41

@Ivo - Pretty much everything in this world has its limits. Carbs are converted to glucose. Excess glucose is stored as fat.
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NickApr 5 '11 at 16:02

@md5sum - Do you experience hitting a wall if you don't eat carbs before cardio? One reason I imagine many people stock up on carbs is because they may not be on a ketogenic diet. Therefore, their body is using glycogen as fuel as opposed to ketone bodies. It may be the case that even while on a ketogenic diet, glycogen stores may be necessary for peak athletic performance. I know that bodybuilders who are trying to gain strength while on a Ketogenic diet have 1 day per week where they "refeed" or "carb up" in order to replenish their glycogen stores in order to be able to lift heavy.
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NickApr 5 '11 at 16:09

Having some carbs before a run definitely boosts my performance.
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Nathan Wheeler♦Apr 5 '11 at 16:11

Athletes carbo load in order to avoid going into glycogen debt during their performance ( essentially avoiding doing your diet ;) The body is designed to use glycogen for energy. Ketogenesis is an emergency backup when that fuel is not available. Its meant to keep your body going which it does but not as efficiently as glycogenisis so you end up depleting your fuel faster than you can make it.

If you want to lose weight, the bottom line will always be calories, so if you're eating extra carbs to help with your runs, you'll have to eat less calories to balance it back out.

As was already stated, fats aren't as readily available for energy as carbohydrates, so they are ideal for low intensity exercise like walking and doing normal daily activities.

I am a bodybuilder (used to be a runner) - The most important time for a bodybuilder to take in carbohydrates is post workout because when the body is depleted it needs to absorb nutrients like amino acids. Carbohydrates increase insulin levels which in turn increases the body's ability to uptake nutrients. So, bodybuilders take in protein with carbohydrates post workout. The second most important time for bodybuilders to take in carbs is pre workout for all the same reasons that a runner takes in carbs before their workout.

Disclaimer...The following is just my loose interpretation of what's going on in your body during a run.

When you take off for a run and are full of nervous energy and maybe a little jumpy, you're running on sugar in the bloodstream. That doesn't last very long before you "settle into a pace" and start burning glycogen that is stored in your muscles when you eat carbohydrates.

When a runner "hits the wall" it feels like a sudden loss of energy as if you can not possibly go any further. I believe this to be the point at which glycogen has become depleted.

If you push past the wall and maintain a strong pace despite all the signals your body is telling you, you'll "get your second wind". At this point, I believe your body has transitioned from burning glycogen to metabolizing protein for energy. When exercising at a high intensity, fat is not as efficient an energy source as muscle tissue, so it is not likely that fat is being burned primarily.

Sometimes you can "hit the wall" again. If you continue to push past that for very long and get a third wind, you'll get "runner's high" which is your body releasing endorphins because that's all you have left to run on.